ContractInterface::get_state_delta() should return a Result

[sanity]

Removed this from get_state_delta() function in interface.rs, not sure if it's still relevant, close if not.

    // FIXME: should return a result type, instead

[iduartgomez]

Yes this still has to be done, but the change is not trivial since requires some juggling with pointers etc. and need to make sure we are making things correctly.

[iduartgomez]

Closing as I am tackling this changes in #167